1. Field of the Invention
The present invention relates to a method and to structure useful in manufacturing solid products by bonding or sintering powder preforms into solid bodies by isostatic hot pressing of the powder under high pressure.
2. Description of the Prior Art
During sintering a powder billet or preform is generally enclosed in a metal casing and is subjected simultaneously to an elevated all encompassing pressure and a high temperature in a pressing furnace. Prior to or in connection with the heating of the preform to said high temperature, the powder preform is normally degassed. Such degassing usually is accomplished at a very low pressure and for this purpose it is conventional to connect the casing which contains the powder body to a vacuum pump.
The present invention is of particular interest in the manufacture of billets of materials such as high-speed type steels or so-called superalloys, which billets may be further machined into a desired shape by rolling, forging, machining or the like. Procedures for the pressure sintering of powder preforms into dense, homogeneous solid bodies are described in greater detail in U.S. Pat. No. 3,893,852, the entirety of which is hereby incorporated herein by reference.
In the past, it has been noted that the heating time for large billets may be as much as 5 to 20 hours. During heating the casing rapidly reaches the highest permissible temperature which may be used for the particular enclosed powder and the casing is then maintained at such temperature to achieve heating throughout the powder in the shortest possible time. The casing must be maintained at these high temperatures for the long periods necessary to achieve sintering of the powder and such intense heat treatment often results in changes in the properties of the casing material. Moreover, when the hot casing is subjected to the effect of oxygen, for example when the hot casing is transferred from a preheat furnace to a pressure furnace, significant changes in casing material properties are often experienced. These changes which often result in leakage during hot pressing operations are severe at or near the nozzle which is generally provided on the casing to facilitate the evacuation of the casing during degassing of the powder preform. Thus, the risk of leakage during hot pressing is particularly great at or near the evacuation nozzle.
In addition to the foregoing, the considerable deformation of the nozzle and the welding of the same during sealing prior to hot pressing often results in grain enlargement in the casing material and cracks initiated at the grain boundaries spread and intensify under subsequent influence of harmful gases such that the casing deteriorates to permit pressurized gas to leak thereinto. Since the nozzle is normally not entirely filled with powder below the point of sealing, when the unfilled nozzle is flattened under the influence of the gas pressure during the hot pressing operation, a great deformation may result leading to fractures and ultimately to leaks through the influence of harmful gases. Needless to say, gas leaking into the casing during hot isostatic pressing will prevent the appropriate compression of the powder and will thus interfere with the complete sintering together of the powder grains. The products thus produced will be unsuitable for use because of the remaining porosity and the same will have to be discarded.